Timer for controlling the operation of photographic enlargers



Patented Dec. 25, 1951 TIMER FOR CONTROLLING THE OPERATION OF PHOTOGRAPHIC ENLARGERS Donald 0. Schwennesen, Chicago, Ill.

Application December 9, 1948, Serial No. 64,360

4 Claims. l

This invention relates to an electrical apparatus and particularly to a timer for controlling the operation of photographic enlargers. Photographic enlargers have become quite popular because of the vogue of small cameras using moving picture i'ilm. As a rule, many of the pictures must be enlarged for enjoyment and for this reason enlargers have gone into extensive use.

As is well known, the quality of an enlargement is determined to a substantial degree by the duration of exposure of the paper. A substantial number of operators of such enlargers are relatively inexperienced or do not use the enlargers often enough to acquire experience. It is therefore desirable to provide a suitable timer which will control the exposure during enlargement and make allowances for variations in light from the projector forming part of the enlarger and also for the density of the negative.

This invention provides a simple and inexpensive timer which may be a separable accessory and added to any enlarger. The invention in general provides a photo-electric cell which receives a selected part or all of the enlarged image as reflected from the surface of the sensitive paper. This photo-electric cell translates the enlarging image into a current of electricity and integrates such current with respect to time. By means of suitable control systems, the time integral of the light impressed upon the photoelectric cell serves to determine the operating cycle of the enlarger.

In order that the invention may be understood it will be explained in connection with the drawing wherein the gure shows a diagrammatic form of a system embodying the invention.

Enlarger IIJ may be of any desired construction having lamp II energized by wires I2 and I3. Enlarger I9 may be provided with any suitable lens system and diaphragm control all well known in the art. Enlarger I is adapted to project an enlarged image upon support I5 carrying suitable enlarging paper for printing. The enlarger and support are well known and require no detailed description.

Rigidly attached to enlarger I9 and movable therewith for focusing and other purposes is an assembly comprising photo-electric cell I6 and lens Il disposed in a suitable casing, not shown. It is understood that the casing protects cell I5 from any light except what passes throughlens system I'I. Lens system I1 is adapted to receive reilected light from part or all of the photonected to junction 3I.

(Cl. Z50- 217) graphic paper on support I5. Lens I'I is adapted to focus an image upon cathode I9 of cell I6.

While photo-electric cell I6 may be any one oi.' a number of types, it is preferably of a type well known on the market wherein an electronic multiplier is combined therewith. Thus for example such a tube, under type number 931A is sold by Radio Corporation of America and by other companies. Such a cell has light sensitive cathode I9 and anode 2| with a plurality of accelerating electrodes 22. The detailed construction of such a photo-electric cell and multiplier are well known and described in numerous patents and publications and no attempt is made to show or describe such construction. The diagrammatic showing of this tube is in accordance with accepted practice as shown in service books of the tube makers.

Accelerating electrodes 22 are connected by wires to taps on secondary 25 of transformer 26.

Cathode I9 of the cell is connected to one end 21 of winding 25 while the other end 28 of this winding is connected by a wire to one terminal of' meter 30. Meter 3| has its other terminal con- Junction 3l is connected by lead 32 to switch contact 33 with which cooperates movable contact 34. Movable contact 34 is connected through suitable resistor 35 to junction 36 on wire 31 going to anode 2I. Junction 3l is connected by wire 38 to junction 39. Between junction 39 and wire 31 is connected condenser 40.

Junction 39 is connected to junction 4I and this ,junction is connected by resistor 42 to switch point 43. Cooperating with switch point 43 is movable switch contact 44. Movable switch contact 44 is mechanically tied to movable switch contact 45 cooperating with switch contact 4B which is dead. Movable contacts 44 and 45 also cooperate with switch contacts 48 and 49 as shown. These movable contacts are biased to this position but may be momentarily moved to the other position for starting exposure cycle. Contact 49 is connected through resistor 59 back to junction 4I. Contact 48 is connected by wire 5I to wire 52. Wire 52 connects cathode 53 of gas tube 54 and grid 55 of this same tube.

Tube 54 is of the type having a thermionic cathode and a gas at a suitable pressure therein and adapted to be triggered by a suitable potential applied to one or more control grids. Such tubes may be energized with alternating current between the cathode and anode and upon the occurrence of a predetermined potential at the control grid, the tube will break down and conduct. As is well known, such gas tubes cannot be controlled once conduction begins unless the anode to cathode circuit is broken. Thus by energizing with alternating current such as 60 cycles, the control grid may regain control 60 times per second. Such grid control tubes are available on the market and are adapted for a vwide variety or' uses.

Tube 54 has its control grid 56 connected to movable switch contacts 49. Tube .54 has anode 58 connected by wire 59 torelayi50. VRelay B0 has its other terminal connected by wire 6| to transformer secondary 62 of transformer 83. The circuit from transformer. secondary 52 continues to junction d then along wire 65 back to cathode 53 of tube 5t. Condenser 66 of suitable capacitance is connected'across the winding of relay |55-Y and. serves to'hold the relaywhen-the samegis energized .by pulsating current.

-Relayr59 has movable contacts V58 and 69 normal-ly biasedto an open andv closed circuit positionrespectively. Thus movable contact 59 of the. relay is connected through resistor to wire 55. Movable contact 58 cooperates with xed contact 1| whichy is connected to wire 12 back to movablecontact 45 of the starting switch. Movablecontact 59 of the relay is connected by wire v to one'terminal of a transformer or other source of potential for enlarger lamp The other'terrninal for this lamp is wire I2 and is connected to the other terminal of this same source of lighting current. Movable contact 69 cooperates with fixed contact 18 which is connected'bywire 19'to wire |3 ofthe enlarger lamp." Cathode 53 of gas tube 515 is connected toresistors 80 and 90 inclusive allv connected in series. Any desiredjunction of'resistors 8| to 90 inclusive may be connected to'wiper contact 92. Resistors 8| to B9 inclusive are preferably logarithmically related with the smallest resistor being 8| and the-'largest'being 90. Resistor t9-is so selected as to provide v:for predetermined operation of `the `gas tube-and condenser charging system. The remai-ningrcondensers are added to modify this Yand provide for a control orV variation. Instead `ofseparate resistors 8| to 90 inclusive may all be fequal and means :may be provided for securingalogarithmic variation of travel of wiper contact. 92.Y along the resistors. Inasmuch as obtaining a logarithmic variation of resistance is wellknown inthe art, and since various methods are known, it. will be assumed that such methods mayvequally well be used.

Wiper Contact 92 is connected back to wire 31 and movable contact 95 of the starter switch. The series ofl resistors 8| to 90 inclusive have their bottomtermi'nal 95 connected to cathode 95 of a two element regulator tube 91. Gas tube 91 is of the type used for maintaining a constant potential and is well adapted to provide voltage regulation. It may be omitted if desired and line voltage be used directly. Thus as an example, tube 91 may be one of the types'of tubes on the market as VR-105. Such a tube is adapted to maintain 'about 105 volts'across its terminals over substantial variations of impressed potential. Tube 91 has anode 98 connected to junction 99. Junction 99 is connected by lead |00 up to junc- .io'n 64 and is also connected through rectifier [9| to junction |02. Junction |02 is connected byjwire'- |03 to'primary l|91! of transformer 63@- Primary |04 has its other terminalconnected to wire |05. Wire |05 is connected by wire I 06 to junction |01. Between junction |01 and junction there is connected resistor |08 while between junction |01 and junction 99 there is connected condenser |09. From junction |51 wire ||0 goes to primary of transformer 25, primary having its other terminal connected by wire H2 to Wire |03. Wires |03 and |05 are connected by switch H5 to any suitable source of alternating current.

The operation of the system is as follows:

-Assume that the system has been deenergized by switch ||5 beingy open. Now if switch I|5 is closed, the system is ready for use. When switch l5 is closed, gas tube 54 starts i'iring since the grid and cathode are at the same potential. The firing circuit may be traced as follows: transformer winding 52, wire 5|, relay winding 60, wire 59, anode 58, cathode 53, wire 55 back to winding 62. Thus relay 60 is energized. The circuit for enlarger`lamp is opened at relay contacts 59 and 18.' At the same time a grid leak circuit is established as follows: control grid 55, wire 12, relay contacts il and 53, resistor 10 to cathode 53. This grid leak circuit maintains the bias of control grid 56 at a constant firing value and prevents accidental timer operation because of leakage currents from the photo-cell. The system as thus described is in a stand-by condition.

If the timer is to be operated for an exposure cycle, the starting switch is momentarily depressed. This moves contact 45 against .contact 49 and at the same time contact 49 moves away from and closes against contact 48.

When contact 55 closes on contact 49,. a discharge circuit including resistor 50 is closed around condenser 40.. The time constant of this circuit is short enough so that condenser 40 is completely discharged by the momentary closurer of the starting switch. At the same time, contacts 134 and 48 completea circuit between control grid 56 and cathode 53. Thus gas tube 54 is maintained in a iiring condition and relayV 60 still remains energized.V

It will be noted that when condenser 40 is connected across` resistor 50, contact 45 drops the potential of both sides of condenser 40 below theV thenetwork, the'more negative that point is to' cathode 53. When the starter switch is released, terminal 39 of condenser i0 is connected to control grid 55 of gas tube'5li, through resistor 42. Hence the control grid potential becomes negative to the cathode and tube 54 ceases firing. This deenergizes relay 59 and the circuit for lamp is closed. This begins an exposure cycle. When the relay releases, the grid leak circuit is'also opened. Light acting upon'photo-cell IG sets up acurrent through the cell. The circuit through the cell may be traced as follows: transformer terminal 28, meter 39, junction 3|, wire 38, condenser 90, wire 31, junction 36, anode 2|, cathode I9 to terminal 21 of the transformer. The polarity of the current charging condenser 40 Vis such as to render junction 39 increasingly positive with respect to wire 31. Thus the potential of control grid 56V gradually rises until it reachesl a firing value. During the charging of condenser 40 by the cell output, the lower condenser terminal (wire 31) remains at a fixed negative potential because of itsconnection to wiper 92.

When1tube5iires, relay 60 will operate and 2X1 tinguish the enlarger lamp.

For testing purposes, switch 34 may be closed against contact 33. This cuts out condenser 40 and the meter registers the rectied cell output current. In the above circuit analysis no attempt has been made to trace the subsidiary multiplier circuits, these being the same as usual.

It is clear that resistor 80 will provide the minimum negative bias for determining an exposure cycle. This may be set to any desired value. As wiper 92 is moved along the resistors away from cathode 53, an increasing initial negative bias is impressed upon control grid 55 of gas tube 54. Thus the lower the initial bias, the longer will condenser 40 have to charge to reach the ring point for gas tube 54. By providing a logarithmic potential divider, the exposure time may be in simple and arithmetical proportion. This, however, may be varied to suit individual requirements. Resistor 42 in the grid circuit of gas tube 54 may be omitted if desired. The same is true of regulator tube 91. This is convenient however since it stabilizes the operation of the voltage divider network including resistors 80 to 90 inclusive.

One example of a system embodying the present invention is as follows: Gas tube 54 may be type 2051 available in the market. Resistor 80 may have a value of 2600 ohms. Resistors 8l to 90 inclusive may have the following values-88; 1200; 1760; 2480; 4020; 4950; 7050; 9950; 14,000 and 4500. Resistor |08 may be 4000 ohms While gas regulator tube 91 may be type 'VR-105. Condensers 40 and |09 may each have a value of 10 microfarads while condenser 60 may have a value of microfarads. Photo-electric cell I 6 was type 931A. The potential developed across secondary 62 was 150 volts. The relay was a standard relay. Resistor 42 had a value of 5 megohms while resistor I0 had a value of 20 megohms. Resistor 50 had a value of 1000 ohms while resistor 35 had a value of 100,000 ohms. Meter 30 and the switch are provided so that the meter may be cut out or cut in to show the current passed by the photo-electric cell. The meter reading is useful to show that the photo-cell is receiving light from a desired part of the enlarger easel.

What is claimed is:

1. A system for controlling the duration of energization of a lamp for photographic exposures, said system comprising a photo-electric cell having cathode and anode, a condenser and source of potential connected in series with said cathode and anode so that said cell controls a charging current to said condenser, said condenser having one terminal whose potential rises positively with respect to the other condenser terminal during charging, .a grid controlled gas tube having thermionic cathode, control grid and anode, a connection including a first position of a switch from said one condenser terminal to said control grid, a source of alternating potential and relay in series connected to said gas tube cathode and anode, a bias resistor having one terminal connected to said gas tube cathode, a

source of direct potential having its positive terminal connected to said gas tube cathode and its negative terminal connected to said other terminal of said bias resistor, a connection between the other terminal of said condenser and a point on said bias resistor, a second switch connected across said condenser for short circuiting said condenser, a connection including a second position oi said lrst named switch for connecting said gas tube control grid and cathode, a lamp energizing circuit including normally engaging contacts for said relay, said two switches being tied together for simultaneous operation with said rst switch normally connecting said one condenser terminal to said gas tube control grid and said second switch normally being open whereby in the `absence 01 light upon said cell, said condenser remains in a discharged condition and said gas tube is in iiring condition with the relay being energized and maintaining the lamp circuit open and upon operation of said switches to an ofinormal position, the one terminal of said condenser is disconnected from the gas tube control grid and connected to the other terminal of said condenser with the condenser being short-circuited and the potential of both terminals thereof being negative to the gas tube cathode, and upon release of said switches back to normal, said control grid of the gas tube is dropped below its cut-off potential to cut the gas tube off, said cell being adapted to gradually charge said condenser and raise the potential of said one condenser terminal and the gas tube control grid to the ring point whereupon said relay is energized and opens said lamp circuit.

2. The circuit of claim 1 wherein a connection including a grid resistor and normally open relay contacts is disposed between the control grid and cathode of said gas tube.

3. 'I'he system according to claim 1 wherein said bias resistor has a plurality of taps thereon with the connection from the lower condenser terminal being made from a potentiometer wiper, said taps being so selected that the variation of bias resistance along the taps is logarithmic in nature.

4. The system according to claim 1 wherein a meter is connected in series with the cell to show cell current and wherein a connection including a switch is provided across the condenser whereby when the switch is closed said condenser is shorted out and the meter may indicate cell current.

DONALD O. SCHWENNESEN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,973,512 Smith Sept. 11, 1934 2,232,373 Dorst Feb. 18, 1941 2,274,158 Penther Feb. 24, 1942 2,443,058 Simmon June 0, 1948 

